Because the original incarnation of the belt drive was a total flop with many issues. Basically, the belt takes a higher tension and loads up the bearings and frame flex was allowing the belt to slip off on the first gen bikes.
Gates redesigned the belt system with a new center track and they now have a frame spec that calls for fairly stiff chain stays (belt stays). The 2nd gen product seems to work, but costs lots more especially if you want to be able to change gearing once in a while. Also, you realize that you have to have a frame made for belts since there has to be a break in either the chainstay or the seat stay on the drive side.

Also chains are 98% efficient and belts are only around 95% efficient at transmitting power and by the time you figure the frame is heavier the overall weight is more. Also, the extra bearing loads might further reduce efficiency, but that would be hard to quantify. The great advantage of the belts is practically zero maintenance and long life.

I was just curious. I saw 1 or 2 belt drive 29ers at my lbs and wondered why no one has converted anything to belt. That makes total sense about the frame thing. Can't break a belt like you can a chain.

One of my single speeds is full rigid and at some point I would probably like to swap that to a belt drive frame. I already like the fact that I don't have to clean the fork after every ride and if it had a belt it would basically be a no maintenance bike. I hardly ever change the gearing and if it was belt I would just never change it. My other SS has a Fox and lighter (aluminum) frame. It is my go fast ss and I would not want a belt on it since I gear it for the course, etc.

This belt subtopic is very interesting.. I suspected as much about the low efficiency, but had no idea about frame stiffness requirements. Wanted a belt-driven Ti rigid SS for the same reason as yourdaguy.

BTW, Frank Schneider's hardtail is built around a Nicolai frame and I guess its rear triangle is very stiff.

Tested my El Mariachi on the trails (the most technical and steep we have) for the first time yesterday. For the first time i realized what riding rigid SS on techical and steep terrain really is. I like it's handling. The bars seemed to be high at first, so i lowered the stem and it felt great both climbing and descending. The climbing felt different from 26'' wheels - it is harder to accelerate so it needs some time to adapt, get familiar with 29er and use it's strengths better (my first 29er). However i haven't ever climbed the stuff that I climbed yesteday on any bike so maybe I'm subjective.
The great thing ridng SS is your ability to see your weak sides. I saw that I'm not strong enough to climb everything so I had to walk from time to time. Another great thing is that SS helps yo to strengthen your weak sides and to become stronger. Riding SS gives you the ULTIMATE workout - it's great both aerobic and anaerobic training, it involves all mscle groups, it improves your balance. What is most important it is allways challenging you: to climb further, to ride more technical trails, to pedal harder, to spin faster. If you like to challenge yourself and to push your self to the limit of your abilities you should love riding SS.
I think I've become an SS addict. I've got 3 SS's: salsa el mariachi for XC (32/18), giant stp ss for jumping and pumptrack (33/16), giant bowery fixed gear for commuting and road training (48/17). I love all of them!

I'm a newbie here.
Just converted my Santa Cruz Highball into a SS., and I now know why gurus here brag so much about SS. The ride is great. I thought the 32-18 gearing was too easy, but it keeps me pumpin all day long up and down the hill at 5km/hr to 30km/hr. I also like the simlicity of all things.

I have found that SSing is much more enjoyable then riding my geared fs. I have less that can go wrong, less to fix, less to maintain. I do not have expensive parts on it at all. I am a cheapo and just want a bike that works. I have zero problems keeping up with my group of riders and can assure you that i have just as much fun with a 1/3 of the problems and a 1/3 of the costs they may incur. I know (short of my chain breaking or taco'ing a wheel) that any problems I am having on the trail have to do with me not my bike.

I still try to grab some air off the spots that can launch me good. I try to hit all the features I can within reason. I might be bombing through the single track but thats just how I ride to keep my flow and momentum for the next nasty climb.

I know little about the newest and greatest gear and dont need it to enjoy what I have. My steel Surly is the best bang for the buck I have ever spent on a bike, although I do miss my '98 Stumpjumper.

Riding SS gives you the ULTIMATE workout - it's great both aerobic and anaerobic training, it involves all mscle groups, it improves your balance.

Absolutely.

On my geared bike I used to sit and spin on the flat bits, sit and spin on the long and not so steep climbs, sit and spin up the semi-hard and hard climb, almost never standing up when climbing and only being able to stand and mash for a very short time before being forced to sit down and spin again. My primary use of standing and mashing was to get a short burst of acceleration in order to sit back down and spin again at a slightly higher cadence than before.

On my ss I sit and spin on the flat bits but since I can't pick a taller gear for more speed I'm slowly improving my max. cadence. I also sit and spin on the long and not so steep climbs, but at a slightly taller gear than I would have picked on my geared bike so I'm building up the power to climb those kind of hills at a higher speed. On the semi-hard and hard climbs I stand and mash. My standing and mashing power and stamina has vastly improved since I started riding ss. I used to only be able to stand and mash for a very short burst, now I can keep going for much longer. Also it makes me use my upper body muscles a lot more.

All in all, as Igoreha says, ss'ing makes a much more complete workout, I can no longer just keep sitting and spinning but have to work on my standing and mashing, my cadence and my upper body too.

After gradually converting from a roadie to a MTBer and then to a SSer I was having a hard time explaining my fitness improvement at 58. I used to ride 5-7,000 road miles a year often competing with young buddies for bragging rights. Yet in the last 6 months since doing almost all of my riding SS, I was losing weight and my resting heart rate dropped from 42 to 40. I was at a loss to explain this since I didn't seem to be working any harder.
Then I read a summery of a paper by a Japanese researcher named Tabata. He found that training at max exertion for 20 second bursts with 10 second rests improved fitness way faster than people that did hours more steady state training. Well the places I ride around here provide an average of 20 seconds of intense climbing and 10 seconds of rest on the way down. Obviously, not all hills are exactly 20 seconds; but overall SS provides a Tabata workout. Knowing this, I choose to ride at places that have these 20 second climbs (many of the places around here are 5-10 seconds and a moment to the bottom. This is insightful information as far as I can tell and has improved my fitness more than swimming, running, racquetball and road riding.

I'm 48 and recovering from knee surgery (torn rear lateral medial meniscus) exactly five weeks ago. I was back on the bike in two weeks, rode the brutal climbing on some of our local trails , and about 95% recovered

I got back into riding at the end of last year to get my fitness up and weight down, and going SS is a great way of benchmarking the progress.

There's this one short, steep hill near the end of my commute home, and because I always tackle it in the same gear I can see just how much stronger I'm getting every week, and know it's not because of gear choices or fine points of cadence, but because I'm mashing it out faster and stronger each time.

took my newly built up 1x1 for my first ever mountain bike ride (singlespeed or otherwise) and really really liked it. Hard work and I had to push a few times but man was it fun. I went out with my buddy riding front sus and 3x8 drivetrain and blazed past him the whole way. Singlespeed mountain biking is quite fun and exciting. I do want to get a larger cog for my rear wheel though.